Ultra low expansion glasses and soft x-ray or extreme ultraviolet (EUV) lithographic elements made from silica and titania traditionally have been made by flame hydrolysis of organometallic precursors of silica and titania. Ultra-low expansion silica-titania articles of glass made by the flame hydrolysis method are used in the manufacture of elements used in mirrors for telescopes used in space exploration and extreme ultraviolet or soft x-ray-based lithography. These lithography elements are used with extreme ultraviolet or soft x-ray radiation to illuminate, project and reduce pattern images that are utilized to form integrated circuit patterns. The use of extreme ultraviolet or soft x-ray radiation is beneficial in that smaller integrated circuit features can be achieved, however, the manipulation and direction of radiation in this wavelength range is difficult. Accordingly, wavelengths in the extreme ultraviolet or soft x-ray range, such as in the 1 nm to 70 nm range, have not been widely used in commercial applications. One of the limitations in this area has been the inability to economically manufacture mirror elements that can withstand exposure to such radiation while maintaining a stable and high quality circuit pattern image. Thus, there is a need for stable high quality glass lithographic elements containing for use with extreme soft x-ray radiation.
One limitation of ultra low expansion titania-silica glass made in accordance with the method described above is that the glass contains striae. Striae are compositional inhomogeneities which adversely affect optical transmission in lens and window elements made from the glass. Striae can be measured by a microprobe that measures compositional variations that correlate to coefficient of thermal expansion (CTE) variations of a few ppb/° C. In some cases, striae have been found to impact surface finish at an angstrom root mean rms level in reflective optic elements made from the glass. Extreme ultraviolet lithographic elements require finishes having a very low rms level.
It would be advantageous to provide improved methods and apparatus for manufacturing ultra low expansion glasses containing silica and titania. In particular, it would be desirable to provide extreme ultraviolet elements having reduced striae and methods and apparatus that are capable of producing such glass elements. In addition, it would be desirable to provide improved methods and apparatus for measuring striae in ultra low expansion glass and extreme ultraviolet lithographic elements.